For years, PWM inverters have been employed to convert DC into AC. In many applications, it is important to carefully regulate the quality of the AC emerging from the inverter. Accordingly, it was customary in the past to define a point of regulation ("POR") downstream of the inverter at which the voltage and current of the AC signal emerging from the inverter would be sampled. The sampled voltage and current would be used to aid in the selection of appropriate PWM switching patterns to minimize distortion at the POR.
Based on the voltage and current sensed at the POR, an inverter controller would select or create in real time an appropriate PWM pattern to ensure the least distortion at the POR. Obviously, as real and reactive power factors of electrical loads coupled to the inverter change, the PWM patterns fed to the inverter would change.
A PWM pattern comprises a set of switching transients which, when applied to a DC signal via an inverter, produces alternating pulses which, when filtered, become a sinusoidal AC signal. The pulses in a PWM pattern are of varying width. Normally, the inverter can adequately reproduce the PWM pattern and thereby create an accurate sinusoidal AC signal from the DC signal. However, under some load conditions (particularly when loads are unbalanced or have particular real or reactive power components), the inverter controller selects a PWM pattern which has switching transients defining pulses which can not be faithfully reproduced by the inverter due to physical limitations of transistor switches therein. In other words, the transistor switches within the inverter have physical limitations which manifest themselves, among other ways, in a minimum allowable switching time. Thus, should the inverter switch be called upon to switch faster than it is able, it will be unable to do so and therefore will be unable to faithfully reproduce a part of the PWM pattern required to maintain the least distortion at the POR.
Prior inventions have addressed schemes for controlling current and voltage at a POR. Representative of such inventions is U.S. Pat. No. 4,757,434, which issued on July 12, 1988 to Kawabata et al. The patent to Kawabata et al is directed to a control circuit for a power conversion apparatus (e.g., inverter and cycloconverter) which produces sinusoidal alternating current (AC) power through multiple switching operations of switching devices within a cycle and supplies the power to a load. The control circuit includes a current control minor loop for controlling the instantaneous value of the output current of the power conversion apparatus in compliance with a current reference value which is determined as the sum of the load current component determined basing on the detected value of load current, and the modification current component produced by a voltage controller to current error of output line voltage of the power conversion apparatus with respect of a sinusoidal voltage reference, whereby the output voltage of the power conversion apparatus is controlled accurately to have less distortion against harmonics of the load.
Kawabata et al is designed to control the output current of the inverter. Kawabata et al fails to show any apparatus for directly dealing with physical switching limitations within the inverter itself. Therefore, Kawabata et al may be subject to the same physical limitations to which the inverter of the subject invention is prone.
The subject invention is the first to provide a DC to AC voltage inverter circuit having a variable voltage DC input such that PWM patterns having short duration switching transients are avoided. Accordingly, minimum switching time physical limitations of the switches within the inverter are avoided.